Are you verifying email addresses? If it's an automatic script it might be the step to throw a kink into their plan.
Make sure you are requiring at least one english letter or number in usernames too, anything to trip them up.
ubb.threads has had (and still does in v7) an http referrer check that makes sure all registrations and posting comes from someone actually being on the site, not from an off-site script. It helps a lot tho you have to warn users that may have problems with it to add your site to their 'safe site list' in their firewall.
